1. Field of the Invention
The present invention relates to a light emitting device, and more particularly to a light emitting device which emits a whitish light with a simplified construction.
2. Prior Art
Japanese patent Laid-Open No. 068166/1999 discloses a light emitting device to emit a light obtained by converting a wavelength of a light emitted from a light-emitting element.
In this light emitting device, a half-mirror (translucence) reflection film is formed on a whole surface of a p-type layer, while a substrate is fixed on a cup portion of a lead frame by a light-transmitting paste including a phosphor.
According to the light emitting device as constructed above, a reflected light of a light going from a light emitting layer to an upper side (a side of p-type layer) by the reflection film and a light going from a light emitting layer to an lower side (a side of n-type layer) are reached the fluorescent paste, in which the wavelength of the light is converted, and then a wavelength-converted light goes to the upper side.
Then, a part of the wavelength-converted light passes through the half-mirror (translucence) reflection film after combining with an inherent light of the light emitting layer, that is, a light, a wavelength of which is not converted, and then it is released outside.
In the conventional light emitting device described above, a color tone of the emitted light can be adjusted freely by changing a reflectance of the half-mirror reflection film. However, a decline of a luminous efficiency can not be avoided, because a part of the light is absorbed by the reflection film.
On the other hand, the reduction of fabrication cost has been required in fabricating of the light emitting device.
Accordingly, it is an object of the invention to provide a light emitting device which is increased in luminous efficiency. It is a further object of the invention to provide a light emitting device which is decreased in fabrication cost without inviting a complicated construction.
According to the first feature of the invention, a light-emitting element includes a light emitting layer for emitting a light of a predetermined wavelength, and p- and n-type electrodes for applying a predetermined voltage across the light emitting layer. The p-type electrode is provided on a light-radiating side of the light emitting layer, and includes a light emitting area for transmitting a light therethrough, and a wire-bonding area to an outside surface of which a bonding wire is bonded and for reflecting a light on an inside surface thereof.
In accordance with a preferred embodiment of the invention, the light emitting area is provided with a predetermined number of opening portions arranged in a predetermined pattern. Each of the predetermined number of opening portions is shaped to be one selected from rectangle, circle, and slit-shape. The light emitting area and the wire-bonding area are equal in thickness to each other.
According to the second feature of the invention, a light emitting device includes a light-emitting element comprising a light emitting layer for emitting a light of a predetermined wavelength and p- and n-type electrodes for applying a predetermined voltage across the light emitting layer, a lead structure including p- and n-leads for mounting the light-emitting element thereon, and a light-transmitting paste containing a phosphor for fixing the light-emitting element to the lead structure. The p-type electrode is provided on a light-radiating side of the light emitting layer, and includes a light emitting area for transmitting a light therethrough and a wire-bonding area to an outside surface of which a bonding wire is bonded and for reflecting a light on an inside surface thereof.
In accordance with a preferred embodiment of the invention, the wire-bonding area reflects a substantially whole portion of the light on the inside surface thereof. The light emitting area is provided with a predetermined number of opening portions arranged in a predetermined pattern. Each of the predetermined number of opening portions is shaped to be one selected from rectangle, circle, and slit-shape. The light emitting area and the wire-bonding area are equal in thickness to each other. The light emitting area and the wire-bonding area are uniform in thickness to be 0.3 xcexcm or more. The light emitting area transmits a whitish light from inside to outside therethrough in accordance with the light of the predetermined wavelength and a light of a converted wavelength emitted from the phosphor which is excited by the light of the predetermined wavelength. The p- and n-leads of the lead structure are p- and n-lead frames, one of the p- and n-lead frames having a cup-shaped portion in which the light-emitting element is mounted via the light-transmitting paste, and the p- and n-type electrodes are connected to the p- and n-lead frames by bonding wires, respectively. The light-emitting element, the bonding wires, and portions of the p- and n-lead frames are sealed by a sealing resin. The sealing resin is shaped to be a lens for focusing a light transmitted through the light emitting area. The p- and n-leads of the lead structure are p- and n-electrode patterns provided on a substrate, one of the p- and n-electrode patterns having an element-mounting portion on which the light-emitting element is mounted via the light-transmitting paste, and the p- and n-type electrodes are connected to the p- and n-electrode patterns by bonding wires, respectively. A reflector having an inner mirror surface of a reverse conical shape for reflecting the light of the predetermined wavelength and a light of a converted wavelength emitted from the phosphor which is excited by the light of the predetermined wavelength. The light-emitting element, the bonding wires, and portions of the p- and n-electrode patterns are sealed in the reflector by a sealing resin.
According to the light emitting device having the above construction, among the light emitted from the light-emitting element, a light going to a side of n-type layer and a reflected light of a light going to a side of p-type layer by the p-type electrode are entered to the light-transmitting paste containing the phosphor (defined as xe2x80x9cfluorescent pastexe2x80x9d hereinafter), in which the wavelength of the light is converted.
The light (defined as xe2x80x9cfluorescent lightxe2x80x9d hereinafter) that its wavelength has been converted is reflected by a cup-shaped portion of the lead frame which is a mounting seat for the light-emitting element, and goes to the p-type electrode, and then a part of that is released from the opening portions of the p-type electrode outside together with the light emitted directly from the light-emitting element. A rest of the fluorescent light is reflected by the p-type electrode, and reflected again by the lead frame (these reflections are repeated), and finally released to the outside from the opening portions of the p-type electrode.
In the light emitting device of the invention, the light emitted from the light-emitting element is not absorbed by the non-opening region of the p-type electrode, because the total light is substantially reflected by the p-type electrode. Therefore, the outside-release-efficiency of the light is improved.
In accordance with a preferred embodiment of the invention, the p-type electrode is a film of a substantially uniform thickness, and has a light emitting area and a bonding area. The light emitting area has optical opening portions for releasing the light to the outside. A conductive wire is bonded to the bonding area. The bonding area is required to be a predetermined film thickness to realize stable wire bonding operation. According to the inventers of the invention, the film thickness of the bonding area is preferably more than 0.3 xcexcm.
In order to simplify a forming step of the p-type electrode, it is preferable to form the light emitting area and the bonding area at the same time. In such a case, the light emitting area and the bonding area are approximately equal in thickness.
As shown in a light emitting device in Japanese patent Laid-Open No. 068166/1999, a p-type electrode of a light-emitting element has a bonding electrode which is additionally provided other than a reflection film. In the present invention, however, the bonding electrode can be omitted to reduce the fabrication cost of the p-type electrode, thereby resulting in a light-emitting element of a low cost which leads to a light emitting device of a low cost.
On the other hand, Japanese patent Laid-Open No. 232450/1994 and Japanese patent Laid-Open No. 237012/1994 disclose p-type electrodes having opening portions. However, each of these p-type electrodes comprises no bonding area.